By Vishnu Satya

The lens quality matter a lot to the images we take, and the viewers view. Having said this There are few key attributes that the lens should have. Often the technical details of the lens are shared. These details includes blades, focal length, glass, T Stop, etc. However, these are important components of the lens which define the images, and aesthetics which are seen and appreciated by the viewer. These need to be tested to push lens characteristics to the maximum. There is numerous test which can be conducted to check these aspects of the lens. The tests for checking these needs to be based on various focal length using ultimate vintage test, Anamorphic Tests, and Full-frame tests with charts, faces, rack focus and flares tests.

Charts are photographed with lens to drill into technical performance, quickly revealing distortions, chromatic aberration, contrast and resolution properties.

Rack Focus reveals breathing and bright fixtures in-frame help reveal chromatic and spherical aberrations.

Depth

The depth information with both in the foreground and background. Dimensionality of the face, as well as color renditioning and vignetting (darkening at the edges of the face). The details of the shades are looked at closely. The effects of the chromatic and spherical aberration would also impact the image depth details.

Closer Look

A closer look at the details. This is a real-world test where the sharpness and the details of the minute expressions of the face is rightly contrasted to get the information of out to the viewers. Hair details, eyebrows and the details of the skin movement which would detail the expressions of the characters. The details of the focus fall-off with the focal distance. Also does the lens add extra pounds to the face or does it make the face look thinner. From these we can find out that different lenses render objects differently. The resolution details also would define the sharpness which are much needed for the close-up shots. Lens can also provide and cooler or warmer skin tone which could change.

As seen in the figure 3 the tranisitons from in-focus to out-of-focus would be much more gradual while in some the fall-off may be quite sudden. Gradual fall-off is considered one of the defining characteristics of a lens.

Bokeh

The Bokeh shape rendering is a characteristic of lens. No two lens are the same based on the Bokeh. The Bokeh of the lens is unique to each lens.  They are also called the “Onion Rings”.  These rings are formed by the lenses aspherical elements. The transitions between the radii of curvature of these elements are difficult to eliminate as a result the ring pattern gets most pronounced. Some lenses have more pronounced rings than the others. One can also see color fringe around the lens because of the residual chromatic aberration.

Round Bokeh

Hexagonal Bokeh

The bokeh could be in circular (fig. 5), and Hexagonal (fig. 6) in shape. A filmmaker could resort to the shape of the choice he wants in an image.

Flare

The Flares are the same as sun or light flares often seen in the piece of glass.  Testing a lens for flares is equally important. The flares various shapes, highlights, and textures result principally from the reflection control of the mechanical and optical elements within the lenses design. The flares is the product of the number and type of glass elements and coating in a lens, the internal treatment of the mechanical components and the camera’s digital sensors.

The flares are a direct result of loss of contrast and light reflecting off edges of elements or internal mechanics such as inside of the barrel itself. One can also see colors (fig. 8) in the flares created by different coatings as seen in the figure 8. Lens designers try to control such flares by using multi-layer anti-reflection coatings. 

The flares are characterized by their movement, shape and quality when the light strikes the lens at different angles of attack. You might have noticed that the flares adds to the mood of the image as much as to its aesthetics.

Focus

The Focus charts that are used to check the focusing capabilities of the lens checks the focus “drop-off” from left to right and from top to bottom. These tests are performed with wide-open aperture. The focus is usually best in the center and fades off as we move away from the center towards the edges as seen in figure 9. The contrast, resolution, and the sharpness improve as we stop down. Contrast, sharpness and vignette all go hand in hand and dependent on each other.

References:

Shot Craft: The shareGrid/Old Glass Ultimate Full-Frame Lens Test by Jay Holben, July 2020